: The rabbit model of Lyme disease allows for the study of B. burgdorferi in the host adapted state. Rabbits infected with Borrelia burgdorferi (Bb) naturally clear the infection within 5 months and are then immune to challenge by both host adapted Borrelia administered by skin implantation and needle injected in vitro cultivated bacteria. Interestingly, in the rabbit model OspA vaccination does not convey protection against host-adapted borrelia and only minimal protection against needle challenge of in vitro cultivated bacteria. Proteins expressed specifically by host adapted Borrelia (HAB) may contribute to protective immunity in the rabbit. However, studies with outer membrane vesicle preparations (OMV) obtained from in vitro cultivated spirochetes indicate that development of protective immunity is multifactorial since vaccination with OMVs also conveys protection at least against needle challenge. Based on these observations the applicants ask the following questions: 1) what level of protection can be achieved by OMV immunization; 2) what role do OMV immunogens play in protective immunity; 3) and what role do proteins selectively expressed by HAB play in infection derived immunity. And lastly they seek to evaluate the unique properties of HAB and define their role in pathogenesis and immunity. To assess these questions they will: a) isolate HAB from rabbit tissue for use in a variety of analyses described below b) they will determine the kinetics of growth of HAB in the rabbit host c) they will use OMV preparations derived from HAB to determine if they confer protective immunity against challenge with both homologous and heterologous strains. If successful these studies will significantly enhance our understanding of protective immunity in the rabbit, allow for the identification of novel immunogens and contribute to efforts to develop Lyme disease vaccines and diagnostic assays.